Globular clusters and blue stragglers

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As shown in Figure 1, Hercules hangs from his knee in the western sky after dusk. This makes it a good time of year to try to spot the globular cluster, M13, with binoculars, or even your unaided eye if you have dark skies and good sight.

Though a small (< half degree) fuzz-ball to us, the Hubble Space Telescope image of its core in Figure 2 reveals a very dense population of stars. Near the center, we would find about 100 stars in a cube 3 light-years on a side. In comparison, the nearest star to the Sun, Proxima Centauri, is four light years away! Any creature on a planet orbiting one of these stars would never see a dark night sky! As I said in the Nov. 17 column, it and other globular clusters, like M92 that you might also seek out with your binoculars, are some of the oldest structures in the Milky Way galaxy. We know this because the majority of stars in them are old and red. Stars, like humans, animals and planets, come into existence, evolve and die. A star will enter its death throes when it runs low on fuel. All stars begin their lives by fusing hydrogen to helium. In this reaction (that scientists have been able to create on Earth, but not sustain), two hydrogen nuclei, protons, combine with electrons to become neutrons, these then bind with two other protons to form a helium nucleus that has two protons and two neutrons. The helium's mass is 0.7% less than that of the four original hydrogens. That tiny amount of mass is converted to energy as described by Einstein's famous equation, E = mc2. Any one reaction only produces a tiny amount of energy, but the Sun fuses a mass of hydrogen equivalent to over 6,000 USS Enterprise aircraft carriers every second!! This is the process by which all stars, including our own Sun, shine. Only a small inner portion of a star like the Sun is dense and hot enough for fusion to take place. With only 20 to 25 percent of the Sun undergoing fusion, its hydrogen fuel will last for about 10 billion years. As stars run out of hydrogen, changes take place that cause them to swell to huge dimensions with the outer layers turning red as the expansion cools them. These are red giant stars. The Sun in this stage may swell to the size of Earth's orbit, engulfing both Mercury and Venus as it does. Within the core, it is possible for Sun-sized stars to muster enough heat in their cores to fuse some helium in to carbon. However, this cannot be sustained for very long and, as they die, they actually blow the outer layers off the dense core to create planetary nebulae such as the Ring Nebula in Lyra, M57. This will be the fate of the Sun in about another 5 billion years. Massive stars have enough heat in the core to fuse heavier and heavier elements until they create a core of iron. Early in the process of fusing multiple elements, they, too, swell into huge red giant stars such as Betelgeuse in the top shoulder of Orion and Aldebaran representing the eye of Taurus the bull. Due to their larger masses, fusion can take place in a larger fraction of these stars and, consequently, they burn through their fuel much more quickly. Betelgeuse, with a mass about 12 times that of the Sun, will have a life span of only about 20 million years and will explode in a spectacular Type II supernova within the next 100,000 years ... “next week” in astronomical terms! This occurs because the iron core can’t be fused into heavier elements so it collapses and causes the rest of the star to explode.

To determine the ages of star clusters, where all the stars were formed at around the same time, astronomers determine their ages by finding the largest mass stars that are still fusing hydrogen and have not begun to run out of fuel. It is by this technique that we have discerned that the Hercules globular cluster is about 11.6 billion years old. So Sun-sized stars are already red giants and evolving toward planetary nebulae.

But there was a puzzle lurking in the globular clusters. In 1953, the astronomer Allan Sandage discovered a class of massive, blue stars while studying M3, a globular cluster in Canes Venatici below the handle of the Big Dipper (unfortunately, this globular cluster sets for us before the sky is dark enough to observe it). Since massive blue stars have short lives, none of them should exist in the 8-billion-year-old cluster. Since they seem to be straggling behind other such massive stars in their evolution, they are called blue stragglers.

One clue to their origin is that blue stragglers are only found in very dense environments, both globular and open clusters (clusters of young stars that formed from the same interstellar cloud … “stellar litters”). Within such environments, stars can interact and even collide. This is the current theory as to how blue stragglers form. As shown in Figure 3, there are two main mechanisms of stellar interactions that could form them. One is the collision of two smaller hydrogen-burning stars, known as “main sequence” stars. Such collisions could occur in clusters but are unlikely in the our more sparsely populated region of the Milky Way. A second mechanism is even more likely. Many stars, perhaps a third of them in our galaxy, are binary stars that orbit each other. Unless they have identical masses, one star will evolve faster than the other and swell into a red giant. At this stage, if the stars are close enough, the red giant star can lose much of its mass to the companion since is so weakly held to its original core. If blue stragglers are created in this way, we should be able to find them to have white dwarf companions that are the naked cores of the former evolved companion.

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In 2015, Natalie Gosnella, now at Colorado College, used the Hubble Space Telescope to study blue stragglers in the open cluster NGC 188. This cluster is near the Little Dipper, but lies within the constellation boundary of Cepheus as shown in Figure 1. Though dimmer than M13, it should be visible in binoculars. Gosnella found that seven of 21 blue stragglers in the cluster had white dwarf companions and seven more show other evidence of mass transfer from binary companions that may now be too faint to be observed.

Every time astronomers think we understand a class of objects, like globular clusters, we find a new puzzle. This is the joy of research, that every question we seem to answer ultimately raises even more questions!

The volunteer astronomers at the Adirondack Public Observatory are eager to show you M13, M92, M57, NGC 188, other Messier “uninteresting fuzzy blurs” and many cosmic wonders. The Roll Off Roof Observatory (RORO) is open to the public on the first and third Fridays of each month approximately one half-hour after sunset. Whether you’re an avid amateur astronomer or have never visited an observatory, come and view through our telescopes and learn about the Wilderness Above. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days, you can also call the RORO at 518-359-6317 to talk with one of our astronomers.

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